The synthesis of heat-shock proteins (hsp) following environmental stress is a highly conserved response and apparently protects cells against long-term effects of adverse conditions. One of the most abundant proteins (P70) in spermatogenic cells is related closely to hsp7O, as shown by ATP affinity chromatography purification, immunoblots of two-dimensional protein gels, and peptide mapping. The P70 protein is synthesized during the meiotic phase of spermatogenesis. Although several members of the hsp7O gene family have been sequenced and their mRNA expression characterized, little is known about the proteins encoded by these genes. We developed antisera specific to P70 and hsp7O to investigate the expression and role of these proteins in male germ cells. The antiserum to P70 was also used to select clones from a mouse spermatocyte cDNA library. One of these clones hybridized to a testes-specific, developmentally regulated 3.0 kb mRNA whose expression was not affected by heat. Sequence analysis indicated that the clone has high homology to previously reported genes of the hsp7O family. However, it appears that spermatogenic cells do not produce hsp7O mRNA or protein in response to heat stress. This suggests that P70 gene expression is linked to suppression of the hsp7O gene in spermatogenic cells. However, the process of Spermatogenesis is unusually sensitive to elevation of temperature and to many toxic agents and P70 may function in germ cell differentiation rather than to protect against the effects of environmental stress. Although P70 expression may Be beneficial to the process of spermatogenesis, this process may also leave male germ cells particularly susceptible to environmental stress.